U.S. patent number 10,359,316 [Application Number 15/617,801] was granted by the patent office on 2019-07-23 for fiber optic bolometer.
This patent grant is currently assigned to NUtech Ventures, Inc., UT-Battelle, LCC. The grantee listed for this patent is NUtech Ventures, Inc., UT-Battelle, LLC. Invention is credited to Ming Han, Matthew L. Reinke.
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United States Patent |
10,359,316 |
Han , et al. |
July 23, 2019 |
Fiber optic bolometer
Abstract
The present disclosure is directed to a fiber optic bolometer
device. In an implementation, a fiber optic bolometer device
includes an optical fiber and a silicon layer that comprises a
Fabry-Perot interferometer. The silicon layer includes a first
surface and a second surface. The fiber optic bolometer device
includes a reflective dielectric film disposed over the first
surface of the silicon layer where the reflective dielectric film
is adjacent to an end face of the optical fiber. The fiber optic
bolometer device also includes an absorptive coating disposed over
the second surface of the silicon layer (e.g., the surface distal
to the end face of the optical fiber).
Inventors: |
Han; Ming (Lincoln, NE),
Reinke; Matthew L. (Rydal, PA) |
Applicant: |
Name |
City |
State |
Country |
Type |
NUtech Ventures, Inc.
UT-Battelle, LLC |
Lincoln
Oak Ridge |
NE
TN |
US
US |
|
|
Assignee: |
NUtech Ventures, Inc. (Lincoln,
NE)
UT-Battelle, LCC (Oak Ridge, TN)
|
Family
ID: |
67300533 |
Appl.
No.: |
15/617,801 |
Filed: |
June 8, 2017 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
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62347148 |
Jun 8, 2016 |
|
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|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G01J
5/0887 (20130101); G01J 5/0853 (20130101); G01N
21/474 (20130101); G01J 5/0896 (20130101); G01N
21/412 (20130101); G01J 5/0821 (20130101); G01J
3/26 (20130101); G01N 2021/4742 (20130101); G01J
2005/583 (20130101); G01N 2021/458 (20130101) |
Current International
Class: |
G01J
5/08 (20060101); G01N 21/41 (20060101); G01N
21/45 (20060101); G01N 21/47 (20060101); G01J
3/45 (20060101); G01B 9/02 (20060101) |
References Cited
[Referenced By]
U.S. Patent Documents
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.
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|
Primary Examiner: Lyons; Michael A
Attorney, Agent or Firm: Greer, Burns & Crain, Ltd.
Fallon; Peter P.
Government Interests
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH AND
DEVELOPMENT
This invention was made with government support under Contract No.
DE-AC05-00OR22725 awarded by the U.S. Department of Energy. The
government has certain rights in the invention.
Parent Case Text
CROSS-REFERENCE TO RELATED APPLICATIONS
The present application claims the benefit under 35 U.S.C. .sctn.
119(e) of U.S. Provisional Application Ser. No. 62/347,148, filed
Jun. 8, 2016, and titled "FIBER OPTIC BOLOMETER." U.S. Provisional
Application Ser. No. 62/347,148 is herein incorporated by reference
in its entirety.
Claims
What is claimed is:
1. A fiber optic bolometer device comprising: an optical fiber; a
silicon layer disposed on an end face of the optical fiber, the
silicon layer comprising a first surface and a second surface,
where the silicon layer comprises a Fabry-Perot interferometer; a
reflective dielectric film disposed over the first surface of the
silicon layer, the reflective dielectric film adjacent to an end
face of the optical fiber; and an absorptive coating disposed over
the second surface of the silicon layer, the second surface distal
to the end face of the optical fiber.
2. The fiber optic bolometer device of claim 1, wherein the optical
fiber comprises at least one of a microstructured fiber, a photonic
crystal fiber, a polarization-maintaining fiber, or a side-hole
fiber.
3. The fiber optic bolometer device of claim 1, wherein the optic
fiber includes an expanded core disposed proximate to the end face
of the optical fiber.
4. The fiber optic bolometer device of claim 1, wherein the silicon
layer includes a silicon pillar.
5. The fiber optic bolometer device of claim 1, wherein the
absorptive coating comprises at least one metal layer.
6. The fiber optic bolometer device of claim 5, wherein the at
least one metal layer comprises at least one of gold or
platinum.
7. The fiber optic bolometer device of claim 1, wherein the optical
fiber includes a cavity disposed along at least a portion of the
optical fiber and proximate to the end face of the optical
fiber.
8. The fiber optic bolometer device of claim 7, wherein the cavity
is at least partially filled with a thermal conductor.
9. The fiber optic bolometer device of claim 1, further comprising
a mirror coating disposed between the optical fiber and the
absorptive coating.
10. The fiber optic bolometer device of claim 9, wherein the mirror
coating is disposed between the silicon layer and the absorptive
coating.
11. The fiber optic bolometer device of claim 1, further comprising
at least one of a graded-index lens or a section of graded-index
fiber disposed between the end face of the optical fiber and the
absorptive coating.
12. A method for fabricating a fiber optic bolometer device,
comprising: receiving an optical fiber including a reflective
dielectric film on a cleaved endface of the optical fiber; forming
at least one silicon pillar; placing an adhesive on the reflective
dielectric film; pressing the optical fiber with the adhesive onto
the silicon pillar; etching the silicon pillar; and placing an
absorptive coating on the silicon pillar.
13. The method for fabricating a bolometer device in claim 12,
wherein the absorptive coating includes gold.
14. A method for demodulating a fiber optic bolometer device array,
comprising: causing a light source to transmit light through an
optical fiber and a circulator to a fiber optic bolometer device
array including at least one fiber optic bolometer device, where
the at least one fiber optic bolometer device includes a silicon
layer and an absorptive layer disposed on an end face of the fiber
optic bolometer device; receiving reflected light from the fiber
optic bolometer device array using at least one photodiode; and
analyzing an output from the at least one photodiode based on
received reflected light to obtain a reflection spectrum of the
bolometer.
15. The method for demodulating a fiber optic bolometer device
array of claim 14, wherein the optical fiber comprises at least one
of a microstructured fiber, a photonic crystal fiber, a
polarization-maintaining fiber, or a side-hole fiber.
16. The method for demodulating a fiber optic bolometer device
array of claim 14, wherein the optic fiber includes an expanded
core disposed proximate to the end face.
17. The method for demodulating a fiber optic bolometer device
array of claim 14, wherein the optical fiber includes a cavity
disposed along at least a portion of the optical fiber and
proximate to the end face of the optical fiber.
18. The method for demodulating a fiber optic bolometer device
array of claim 17, wherein the cavity is at least partially filled
with a thermal conductor.
19. The method for demodulating a fiber optic bolometer device
array of claim 14, further comprising a mirror coating disposed
between the optical fiber and the absorptive coating.
20. The method for demodulating a fiber optic bolometer device
array of claim 14, further comprising at least one of a
graded-index lens or a section of graded-index fiber disposed
between the end face of the optical fiber and the absorptive
coating.
Description
BACKGROUND
An optical fiber can include a flexible, transparent fiber made of
extruded glass (silica) or plastic. Light can be transmitted
between two ends of the optical fiber, which may be used in
fiber-optic communications. A fiber optic sensor uses an optical
fiber either as the sensing element (e.g., an intrinsic sensor) or
as a means of relaying signals from a remote sensor to electronics
that process a signal within the optical fiber (e.g., an extrinsic
sensor). Fiber-optic sensors, such as intrinsic sensors, utilize
optical fibers to measure temperature, strain, pressure, and/or
other characteristics associated with the optical fiber.
SUMMARY
A fiber optic bolometer device is described herein. In an
implementation, a fiber optic bolometer device includes an optical
fiber and a silicon layer that comprises a Fabry-Perot
interferometer. The silicon layer includes a first surface and a
second surface. The fiber optic bolometer device includes a
reflective dielectric film disposed over the first surface of the
silicon layer where the reflective dielectric film is adjacent to
an end face of the optical fiber. The fiber optic bolometer device
also includes an absorptive coating disposed over the second
surface of the silicon layer (e.g., the surface distal to the end
face of the optical fiber). This absorptive coating may also serve
as a mirror of the Fabry Perot interferometer.
This Summary is provided to introduce a selection of concepts in a
simplified form that are further described below in the Detailed
Description. This Summary is not intended to identify key features
or essential features of the claimed subject matter, nor is it
intended to be used as an aid in determining the scope of the
claimed subject matter.
DRAWINGS
The detailed description is described with reference to the
accompanying figures. The use of the same reference numbers in
different instances in the description and the figures may indicate
similar or identical items.
FIG. 1A is an isometric view illustrating an embodiment of a fiber
optic sensor that includes a silicon layer disposed on an end face
of an optical fiber, in accordance with an example implementation
of the present disclosure.
FIG. 1B is a partial side elevation cross section view illustrating
an embodiment of a fiber optic sensor that includes multiple
cascaded silicon layers disposed on an end face of an optical
fiber, in accordance with an example implementation of the present
disclosure.
FIG. 1C is a partial side elevation cross section view illustrating
an embodiment of a fiber optic sensor that includes multiple
cascaded silicon layers disposed on an end face of an optical
fiber, in accordance with an example implementation of the present
disclosure.
FIG. 1D is an environmental view illustrating an embodiment of a
fiber optic sensing system that includes a fiber optic sensor with
a silicon layer disposed on an end face of an optical fiber, in
accordance with an example implementation of the present
disclosure.
FIG. 1E is an environmental view illustrating an embodiment of a
fiber optic sensing system that includes a fiber optic sensor with
a silicon layer disposed on an end face of an optical fiber, in
accordance with an example implementation of the present
disclosure.
FIG. 1F is an environmental view illustrating an embodiment of a
controller used in a fiber optic sensing system, in accordance with
an example implementation of the present disclosure.
FIG. 1G is an isometric view illustrating an embodiment of a fiber
optic sensor that includes a silicon layer disposed on an end face
of an optical fiber, in accordance with an example implementation
of the present disclosure.
FIG. 1H is a graphical depiction illustrating a wavelength shift
that represents a temperature change using a fiber optic sensor
that includes a silicon layer disposed on an end face of an optical
fiber, in accordance with an example implementation of the present
disclosure.
FIG. 1I is a side elevation cross sectional view illustrating an
embodiment of a fiber optic sensor that includes a silicon layer
disposed on an end face of an optical fiber, in accordance with an
example implementation of the present disclosure.
FIG. 1J is a side elevation cross sectional view illustrating an
embodiment of a fiber optic bolometer sensor that includes a
silicon layer disposed on an end face of an optical fiber where the
silicon layer includes a reflective dielectric film disposed over a
first surface and an absorptive and reflective coating disposed
over a second surface in accordance with an example implementation
of the present disclosure.
FIG. 1K is a side elevation cross sectional view illustrating an
embodiment of fiber optic bolometer sensor in accordance with an
example implementation of the present disclosure.
FIG. 1L is a side elevation cross sectional view illustrating an
embodiment of fiber optic bolometer sensor in accordance with an
example implementation of the present disclosure.
FIG. 1M is a side elevation cross sectional view illustrating an
embodiment of fiber optic bolometer sensor in accordance with an
example implementation of the present disclosure.
FIG. 1N is a side elevation cross sectional view illustrating an
embodiment of fiber optic bolometer sensor in accordance with an
example implementation of the present disclosure.
FIG. 1O is a side elevation cross sectional view illustrating an
embodiment of fiber optic bolometer sensor in accordance with an
example implementation of the present disclosure.
FIG. 2A is a flow diagram illustrating an example process for using
a fiber optic sensor that includes a silicon layer disposed on an
end face of an optical fiber, such as the fiber optic sensor
illustrated in FIGS. 1A through 1G.
FIG. 2B is a flow diagram illustrating an example process for
fabricating a fiber optic sensor that includes a silicon layer
disposed on an end face of an optical fiber, such as the fiber
optic sensor illustrated in FIGS. 1A through 1G.
FIG. 2C is a flow diagram illustrating an example process for
fabricating a fiber optic bolometer device that includes a silicon
layer disposed on an end face of an optical fiber, such as the
fiber optic sensor illustrated in FIGS. 1J through 1M.
FIG. 2D is a flow diagram illustrating an example process for
demodulating an array of fiber optic bolometer devices that
includes a silicon layer disposed on an end face of each optical
fiber, such as the fiber optic bolometer device illustrated in
FIGS. 1J through 1M.
FIG. 3A is a diagrammatic partial cross-sectional side elevation
view illustrating the fabrication of a fiber optic sensor that
includes a silicon layer disposed on an end face of an optical
fiber, such as the fiber optic sensor illustrated in FIGS. 1A
through 1G, in accordance with the process shown in FIG. 2B.
FIG. 3B is a diagrammatic partial cross-sectional side elevation
view illustrating the fabrication of a fiber optic sensor that
includes a silicon layer disposed on an end face of an optical
fiber, such as the fiber optic sensor illustrated in FIGS. 1A
through 1G, in accordance with the process shown in FIG. 2B.
FIG. 3C is a diagrammatic partial cross-sectional side elevation
view illustrating the fabrication of a fiber optic sensor that
includes a silicon layer disposed on an end face of an optical
fiber, such as the fiber optic sensor illustrated in FIGS. 1A
through 1G, in accordance with the process shown in FIG. 2B.
FIG. 3D is a diagrammatic partial cross-sectional side elevation
view illustrating the fabrication of a fiber optic sensor that
includes a silicon layer disposed on an end face of an optical
fiber, such as the fiber optic sensor illustrated in FIGS. 1A
through 1G, in accordance with the process shown in FIG. 2B.
FIG. 3E is a diagrammatic partial cross-sectional side elevation
view illustrating the fabrication of a fiber optic sensor that
includes a silicon layer disposed on an end face of an optical
fiber, such as the fiber optic sensor illustrated in FIGS. 1A
through 1G, in accordance with the process shown in FIG. 2B.
FIG. 4A is a diagrammatic partial cross-sectional side elevation
view illustrating the fabrication of a fiber optic bolometer device
that includes a silicon layer disposed on an end face of an optical
fiber, such as the fiber optic bolometer device illustrated in
FIGS. 1J through 1M, in accordance with the process shown in FIG.
2C.
FIG. 4B is a top plan view illustrating the fabrication of a fiber
optic bolometer device that includes a silicon layer disposed on an
end face of an optical fiber, such as the fiber optic bolometer
device illustrated in FIGS. 1J through 1M, in accordance with the
process shown in FIG. 2C.
FIG. 4C is a diagrammatic partial cross-sectional side elevation
view illustrating the fabrication of a fiber optic bolometer device
that includes a silicon layer disposed on an end face of an optical
fiber, such as the fiber optic bolometer device illustrated in
FIGS. 1J through 1M, in accordance with the process shown in FIG.
2C.
FIG. 4D is a diagrammatic partial cross-sectional side elevation
view illustrating the fabrication of a fiber optic bolometer device
that includes a silicon layer disposed on an end face of an optical
fiber, such as the fiber optic bolometer device illustrated in
FIGS. 1J through 1M, in accordance with the process shown in FIG.
2C.
FIG. 4E is a diagrammatic partial cross-sectional side elevation
view illustrating the fabrication of a fiber optic bolometer device
that includes a silicon layer disposed on an end face of an optical
fiber, such as the fiber optic bolometer device illustrated in
FIGS. 1J through 1M, in accordance with the process shown in FIG.
2C.
FIG. 4F is a diagrammatic partial cross-sectional side elevation
view illustrating the fabrication of a fiber optic bolometer device
that includes a silicon layer disposed on an end face of an optical
fiber, such as the fiber optic bolometer device illustrated in
FIGS. 1J through 1M, in accordance with the process shown in FIG.
2C.
FIG. 5 is an environmental view illustrating an embodiment of a
fiber optic bolometer device array that includes multiple fiber
optic bolometer devices with a silicon layer and an absorptive
layer disposed on an end face of an optical fiber, in accordance
with an example implementation of the present disclosure.
DETAILED DESCRIPTION
Measurement of speed of gas or liquid flow is of great practical
importance in a variety of industries, such as food inspection,
pharmacy, oil/gas exploration, environmental, high-voltage power
systems, chemical plants, and oceanography research. Owing to their
many unique advantages, such as small size, light weight, immunity
to electromagnetic interference, remote sensing capability, harsh
environment tolerance, and capability for distributed or
quasi-distributed measurement fiber-optic sensors, such as
temperature sensors, flowmeters, or anemometers, have proven to be
attractive alternatives to their traditional mechanical or
electromagnetic counterparts.
In addition to sensitivity and temperature range, two important
sensor parameters include temperature resolution and speed (or
response time). Temperature resolution, defined as the minimum
detectable temperature changes, is determined by both the sensor
sensitivity (defined as the sensor output from a given temperature
change) and the noise of the sensor system, while the response time
is mostly limited by the time constant of the heat transfer process
between the sensing element and the surrounding environment. The
sensing element of many fiber-optic temperature sensors is part of
the fiber itself, which is made of fused silica. The temperature
resolution and the speed are limited by the relatively low
thermo-optic coefficient (TOC) and thermal diffusivity of the glass
material that lead, respectively, to a reduced sensor sensitivity
and increased time for the temperature of the sensing element to
reach equilibrium with the surrounding environment. For example, it
is well-known that a fiber Bragg grating (FBG), whose reflection
spectrum features a single reflection peak, exhibit a temperature
sensitivity of about 10 pm/.degree. C. A fiber modal interferometer
based on a single mode-multimode-single mode fiber structure has
been reported to have a temperature resolution of 0.2.degree. C.
Many all-silica-fiber-based temperature sensors possess relatively
low sensitivity and relatively low temperature resolution. As to
the response time, a package of a FBG with a copper tube
encapsulation can greatly reduce the response time of the sensor
from several seconds to 48.6 milliseconds (ms) in water. A response
time of 16 ms in air has been demonstrated for a microfiber coupler
tip temperature sensor.
Compared to fused silica, crystalline silicon is a much more
desirable sensor material for high-resolution and high-speed
temperature sensing. Silicon is highly transparent over the
infrared wavelength and has a TOC approximately 10 times larger
than that of fused silica used for the sensing element for most
fiber-optic sensors, resulting in potentially much higher
temperature sensitivity. In addition, a silicon-based temperature
sensor also has high speed because of the large thermal diffusivity
of silicon, which is comparable to many metals (e.g., aluminum and
gold) and is more than 60 times larger than fused silica. However,
the use of silicon as a temperature sensing element has not been
utilized on a large scale for high-resolution and high-speed
temperature sensing. The dependence of the absorption of a silicon
film on temperature for temperature sensing and the sensor has
shown a relatively low temperature resolution of .+-.0.12.degree.
C. and a long response time on the order of 1 second (s). A simpler
structure with a thin silicon film (thickness <1 .mu.m)
deposited directly on the fiber end through electron-beam
evaporation has shown a temperature resolution of only 3.degree. C.
In this case, radio-frequency sputtering was applied to simplify
the deposition process, and the resolution was mainly limited by
the small thickness of the silicon film that led to broad spectral
fringes. Instead of silicon film, a silicon micro-waveguide
patterned on a micro-electro-mechanical system (MEMS) was developed
as temperature sensor, and due to the increased length of the Si
sensing element, the temperature resolution was improved to
0.064.degree. C. However, it is a challenge to integrate the fiber
and the MEMS into a single sensor device, and the large size of
previous sensing elements also limit their temperature measurement
speed.
Accordingly, a fiber optic sensor, a process for utilizing a fiber
optic sensor, and a process for fabricating a fiber optic sensor
are described, where a double-side-polished silicon pillar is
attached to an optical fiber tip and forms a Fabry-Perot (FP)
cavity and a sensor head. As described herein, the Fabry-Perot (FP)
cavity may comprise a Fabry-Perot (FP) cavity bolometer (e.g., a
fiber-optic bolometer device or a fiber-optic bolometer sensor).
The bolometer is configured to measure the power of incident
electromagnetic radiation via the heating of a material with a
temperature-dependent electrical resistance.
In an implementation, a fiber optic bolometer device includes an
optical fiber and a silicon layer that comprises a Fabry-Perot
interferometer. The silicon layer includes a first surface and a
second surface. The fiber optic bolometer device includes a
reflective dielectric film disposed over the first surface of the
silicon layer where the reflective dielectric film is adjacent to
an end face of the optical fiber. The fiber optic bolometer device
also includes an absorptive coating disposed over the second
surface of the silicon layer (e.g., the surface distal to the end
face of the optical fiber).
Example Implementations
FIGS. 1A through 1N illustrate a fiber optic sensor 100 (fiber
optic bolometer device 100) and fiber optic sensing system 130 in
accordance with an example implementation of the present
disclosure. The fiber optic sensor 100 can include an optical fiber
102 configured to be coupled to a light source 126 and a high speed
spectrometer 128. The fiber optic sensor 100 and fiber optic
sensing system 130 may be utilized as a temperature sensor in
determining temperature in gas and liquid.
In implementations, the optical fiber 102 can include a flexible,
transparent fiber core 104 made of extruded glass (e.g., silica) or
a polymer. The optical fiber 102 can be configured to transmit
light between the two ends of the optical fiber 102. In some
instances, the optical fiber 102 may be immune to electromagnetic
interference.
The optical fiber 102 can include the core 104 and/or a cladding
106. The core 104 may include a fiber of glass and/or plastic that
extends along the length of the optical fiber 102. The core 104 may
be surrounded by a cladding 106, which may include a material with
a lower index of refraction than the core 104. In embodiments, the
cladding 106 may include a cladding of a different glass and/or
plastic, a buffer layer, and/or a jacket.
In some instances, the polarization of the light in regular
single-mode optical fibers may not be stable. Mechanical or thermal
perturbations to the optical fiber 102 may change the polarization.
Due to the potential birefringence of the silicon layer 108 and/or
the reflective dielectric thin film 110, the optical length of the
FPI may be dependent on the light polarization. The changes of the
polarization may lead to the change of the optical signal, which
may not be distinguished from the change from the radiation. To
eliminate the issues from the birefringence of the FPI, some
embodiments of the fiber optic sensor 100 may include an optic
fiber 102 comprising polarization-maintaining fibers (PMFs), where
the polarization of the light beam in the fiber is maintained
during the light propagation in the fiber.
As illustrated in FIG. 1A, the fiber optic sensor 100 can include a
silicon layer 108 disposed on an end face 152 of the optical fiber
102 (e.g., a cleaved portion of the optical fiber 102), which forms
a sensor head. In implementations, the silicon layer 108 can
include a silicon pillar and/or a silicon-based film. In a specific
embodiment, the silicon layer 108 includes a double-sided polished
silicon pillar. In another specific embodiment, the silicon layer
108 can include a piece of a silicon wafer bonded to the end face
152. The silicon layer 108 may include various diameters and/or
lengths. For example, the silicon layer 108 can include a silicon
pillar with a diameter between about 80 .mu.m to 100 .mu.m with a
length of about 200 pm. In another example, the silicon layer 108
may include a piece of silicon that is approximately 10 .mu.m
thick. In yet another example, the silicon layer 108 can include a
piece of silicon that is approximately 200 .mu.m thick. It is
contemplated that the silicon layer 108 may include other diameters
and/or lengths. The silicon layer 108 diameter is generally less
than the diameter of the optical fiber 102, which leads to a fast
temperature response. The silicon in the silicon layer 108 is
highly transparent over the infrared wavelength and has a TOC
approximately 10 times larger than that of the silica used in the
optical fiber 102 for most fiber-optic sensors, resulting in
potentially much higher temperature sensitivity. In addition, the
fiber optic sensor 100 also has high speed because of the large
thermal diffusivity of silicon, which is comparable to many metals
(e.g., aluminum and gold) and more than 60 times larger than fused
silica. In other implementations, the silicon layer 108 may be
replaced with other materials that have large thermal diffusivity
and high thermo-optic and thermal expansion coefficients. Further,
it is contemplated that the silicon layer 108 can include other
configurations, such as a cuboid configuration.
As illustrated in FIGS. 1B and 1C, the fiber optic sensor 100 may
include cascaded Fabry-Perot cavities. In these implementations,
the fiber optic sensor 100 can include an adhesive 156 disposed on
an end face 152 of the optical fiber 102, a silicon layer 108
disposed on the adhesive 156, a second adhesive 166 disposed on the
silicon layer 108, and a second silicon layer 168 disposed on the
second adhesive 166. The silicon layer 108 and the second silicon
layer 168 include the same material so that each has the same
responsivity to temperature. In such a way, the large free spectrum
range (FSR) of the envelope originating from the second silicon
layer 168 (the second Fabry-Perot interferometer) provides large
dynamic range, while the recognized dense fringes with small FSR
stemming from the silicon layer 108 (the first Fabry-Perot
interferometer) offers high resolution due to the narrow fringes.
The adhesive 156 and/or the second adhesive 166 can include
materials suitable to bond the optical fiber 102, the silicon layer
108, and/or the second silicon layer 168 (e.g., a UV glue, etc.).
Additionally, the adhesive 156 and/or the second adhesive 166 can
be the same or similar diameter as the silicon layer 108 and/or
second silicon layer 168, while the thickness of the adhesive 156
and/or the second adhesive 166 can be only a few microns (e.g.,
<1 .mu.m, 2 .mu.m, 3 .mu.m, etc.). Because the adhesive 156
and/or the second adhesive 166 are very thin compared to the
silicon layer 108 and the second silicon layer 168, they show
negligible influence on the reflection spectrum of reflected light
162. In specific embodiments, the silicon layer 108 and the second
silicon layer 168 can be the same or similar diameters but have
different lengths (e.g., the silicon layer 108 is 200 .mu.m in
length and the second silicon layer 168 is 10 .mu.m in length). It
is contemplated that the silicon layer 108 and/or the second
silicon layer 168 can include a variety of lengths and/or
diameters. In FIG. 1C, n.sub.i and D.sub.i represent the refractive
index and separation of the i.sup.th layer, respectively. These
implementations provide an optical fiber thermometer based on
double cascaded Fabry-Perot interferometers both made from the same
material of silicon but with vastly different cavity lengths to
achieve both large dynamic and high resolution.
In implementations, the fiber optic sensor 100 defines and includes
a Fabry-Perot (FP) cavity. A Fabry-Perot cavity (or Fabry-Perot
interferometer) can include a cavity formed by the optical fiber
102 and the silicon layer 108 disposed on the end face 152 of the
optical fiber 102. Due to the thermo-optic effect, temperature
variations change the optical thickness of the FP cavity and
consequently cause spectral shifts in its reflection spectrum.
As illustrated in FIG. 1D, the fiber optic sensing system 130 can
include the fiber optic sensor 100, a light source 126, a
circulator 124, a spectrometer 128, and a controller 132. In some
implementations, the fiber optic sensing system 130 may include a
heating light source 146.
In implementations, the light source 126 (e.g., a broad band
source) transmits light to the circulator 124 and the fiber optic
sensor 100. In one specific embodiment, light source 126 includes a
wavelength swept laser, such as a high-speed, narrow-linewidth, and
wavelength-swept laser. In another specific embodiment, light
source 126 includes a laser diode. In yet another specific
embodiment, light source 126 includes a white light source (e.g.,
1550 nm). It is contemplated that the light source 126 can include
other types of light sources. In implementations, the light source
126 is optically coupled to the optical fiber 102, which is
optically coupled to a circulator 124. Additionally, the light
source 126 can be coupled to and controlled using controller
132.
As illustrated in FIG. 1E, the fiber optic sensing system 130 may
include a heating light source 146 configured to provide heating
light 142. In these embodiments, the heating light source 146 can
include a light source, such as a red laser diode, that is
optically coupled to the optical fiber 102 using a coupler 164. In
one specific instance, the heating light source 146 can include a
635 nm diode laser. The heating light source 146 may include other
light sources that provide light, which can be absorbed by the
fiber optic sensor 100 and/or the silicon layer 108. Additionally,
the heating light source 146 can be controlled using controller
132.
A circulator 124 can include a fiber-optic component used to
separate optical signals in optical fiber 102. In implementations,
circulator 122 can direct transmitted light 140 from light source
126 (and/or heating light 142 from heating light source 146) to
fiber optic sensor 100 while directing reflected light 162 from the
fiber optic sensor 100 to spectrometer 128.
The fiber optic sensing system 130 can include a spectrometer 128
coupled to the optical fiber 102 and a controller 132. In
implementations, a spectrometer 128 can include a light sensor,
such as a photodetector, configured to detect reflected light 162
and the associated spectra from the optical fiber 102 and fiber
optic sensor 100. In a specific embodiment, the spectrometer 128
may include a high-speed photodetector (e.g., the high speed
spectrometer from Ibsen Photonics, I-MON 256 USB, Denmark).
Additionally, the spectrometer 128 can be coupled to and controlled
using controller 132.
As reflected light 162 is received and/or detected by spectrometer
128, a shift in wavelength is detected when temperature changes at
the silicon layer 108. The wavelength of the N.sup.th fringe peak,
.lamda..sub.N, of the reflection spectrum is given as
.times..lamda..times..times..times. ##EQU00001## where n and L are,
respectively, the RI and cavity length of the FP cavity. Both n and
L are dependent on temperature due to the thermo-optic effect and
the thermal expansion of the silicon material. Therefore,
temperature change can be measured by monitoring .lamda..sub.N.
From Eq. 1, the temperature sensitivity is given by
.differential..lamda..differential..lamda..function..times..differential.-
.differential..times..differential..differential..times.
##EQU00002## Although Eqs. 1 and 2 only depict one of the multiple
peaks in the reflected spectrum from the sensor, in some cases, an
average wavelength may be applied to significantly reduce the noise
lever or increase the resolution. This average wavelength can be
obtained from multiple peaks or valleys or both.
The sensitivity depends on the TOC and the thermo-expansion
coefficient (TEC) of the sensing material. The TOC and TEC for
silicon are, respectively, 1.5.times.10.sup.-4 RIU/.degree. C. and
2.55.times.10.sup.-6 m/(m.degree. C.) at 25.degree. C. To estimate
the sensitivity, these values are applied to Eq. (2) at the peak
wavelength .lamda..sub.N around 1550 nm and the RI of silicon is
assumed to be 3.4. From this, the sensitivity of the temperature
sensor proposed here is estimated to be 72 pm/.degree. C. As a
comparison to the all-fiber based sensor, the TOC and TEC for fused
silica are, respectively, 1.28.times.10.sup.-5 RIU/.degree. C. and
5.5.times.10.sup.-7 m/(m.degree. C.) at 25.degree. C., both of
which are much smaller than those for silicon. Assuming the RI of
silica at 1550 nm is 1.5, the sensitivity of an all-fiber based
sensor is about 14 pm/.degree. C., which is more than 5 times
smaller than the fiber optic sensor 100.
The high RI (about 3.4) of silicon over infrared wavelength range
produces a high reflectivity at the interfaces between silicon
layer 108 and the surrounding environment and between silicon layer
108 and the fiber end face 152, which facilitates to obtain a large
optical power and a high fringe-visibility of the interferometric
spectrum from the FP cavity for improving the sensor resolution. In
addition, the high RI and the relatively long FP cavity yield a
large number of fringes within the wavelength range of the
spectrometer, which can be exploited to further increase the
temperature resolution.
The fiber optic sensor 100 also features a short response time. Due
to the high thermal diffusivity of silicon and the small size of
the sensor head, the temperature within the FP cavity can quickly
reach equilibrium with surroundings.
As illustrated in FIG. 1B, the fiber optic sensing system 130 can
include a controller 132 that is configured to determine a shift in
spectra detected by spectrometer 128 using a fiber optic sensor
100. The controller 132 can be coupled to the components of the
fiber optic sensing system 130. Additionally, the controller 132
may be configured in a variety of ways. As illustrated in FIG. 1F,
the controller 132 is illustrated as including a processor 134, a
memory 136, and a communications interface 138. The processor 134
provides processing functionality for the fiber optic sensor 100
and may include any number of processors, micro-controllers, or
other processing systems, and resident or external memory for
storing data and other information accessed or generated by the
fiber optic sensor 100. The processor 134 may execute one or more
software programs that implement the techniques and modules
described herein. The processor 134 is not limited by the materials
from which it is formed or the processing mechanisms employed
therein and, as such, may be implemented via semiconductor(s)
and/or transistors (e.g., electronic integrated circuits (ICs)),
and so forth.
The memory 136 is an example of a non-transitory computer storage
device that provides storage functionality to store various data
associated with the operation of the fiber optic sensor 100, such
as the software program and code segments mentioned above, computer
instructions, and/or other data to instruct the processor 134 and
other elements of the fiber optic sensor 100 to perform the
techniques described herein. Although a single memory 136 is shown,
a wide variety of types and combinations of memory may be employed.
The memory 136 may be integral with the processor 134, stand-alone
memory, or a combination of both. The memory may include, for
example, removable and non-removable memory elements such as RAM,
ROM, Flash (e.g., SD Card, mini-SD card, micro-SD Card), magnetic,
optical, USB memory devices, and so forth.
The communications interface 138 is operatively configured to
communicate with components of the fiber optic sensor 100. For
example, the communications interface 138 can be configured to
transmit data for storage in the controller 132, retrieve data from
storage in the controller 132, and so forth. The communications
interface 138 is also communicatively coupled with the processor
134 to facilitate data transfer between components of the fiber
optic sensing system 130 and the processor 134 (e.g., for
communicating inputs to the processor 134 received from a device
communicatively coupled with the fiber optic sensing system 130).
It should be noted that while the communications interface 138 is
described as a component of fiber optic sensing system 130, one or
more components of the communications interface 138 can be
implemented as external components communicatively coupled to the
fiber optic sensing system 130 via a wired and/or wireless
connection. The fiber optic sensing system 130 can also comprise
and/or connect to one or more input/output (I/O) devices (e.g., via
the communications interface 138) including, but not necessarily
limited to a display, a mouse, a touchpad, a keyboard, and so
on.
The communications interface 138 and/or the processor 134 can be
configured to communicate with a variety of different networks
including, but not necessarily limited to: a wide-area cellular
telephone network, such as a 3G cellular network, a 4G cellular
network, or a global system for mobile communications (GSM)
network; a wireless computer communications network, such as a WiFi
network (e.g., a wireless local area network (WLAN) operated using
IEEE 802.11 network standards); an internet; the Internet; a wide
area network (WAN); a local area network (LAN); a personal area
network (PAN) (e.g., a wireless personal area network (WPAN)
operated using IEEE 802.15 network standards); a public telephone
network; an extranet; an intranet; and so on. However, this list is
provided by way of example only and is not meant to be restrictive
of the present disclosure. Further, the communications interface
138 can be configured to communicate with a single network or
multiple networks across different access points.
In one specific embodiment illustrated in FIGS. 1G through 1I, the
fiber optic sensor 100 can function as a fiber optic anemometer. In
this embodiment, the transmitted light 140 (e.g., white-light
centered at 1550 nm) is injected through the optical fiber 102 to
the FP defined in the fiber optic sensor 100 by the optical fiber
102 and the silicon layer 108, and the reflection spectrum of the
reflected light 162 can be recorded by a high-speed spectrometer
128. At the same time, heating light 142 from a heating light
source 146 (e.g., 635 nm diode laser) can be guided through the
same optical fiber 102 to the heat the FP. Silicon has a band gap
energy of 1.11 eV and is highly transparent to the transmitted
light 140 but is opaque to the heating light 142. Therefore, the FP
temperature can be effectively increased by the heating light 142.
When air moves (e.g., air convection 144) over the surface of a hot
silicon layer 108, a cooling effect from the moving air reduces the
temperature of the silicon layer 108 and the FP and introduces a
shift to the fringe valley wavelength of the reflection spectrum,
as schematically shown in FIGS. 1H and 1I. The wavelength shift can
be separated by the spectrometer 128 and/or the controller 132 into
a wind-temperature-induced wavelength shift and a
wind-speed-induced wavelength shift. As a result, temperature
self-compensated measurement of wind speed can be achieved by
comparing the shift in the wavelengths of a fringe valley when the
heating laser is turned on and off to determine
temperature-compensated wind speed. It should be pointed out that
although it is implemented as an anemometer in this example, it is
not limited to measuring only the wind or air flow. Any other kind
of flows (e.g., water flow) that can bring about the cooling
effects to the heated sensor head can be measured.
FIG. 1J illustrates an embodiment of a fiber-optic sensor 100 that
comprises a bolometer. As described above, a bolometer measures or
detects power flux from electromagnetic radiation or particles by
the heating of the respective material. In this embodiment, a
Fabry-Perot interferometer (FPI) is coupled to the end face 152 of
an optical fiber 102. The FPI can be formed by a silicon pillar
(e.g., silicon layer 108) with a first side of the silicon pillar
coated with an absorptive and reflective layer (e.g., coating) 112
and the other side (e.g., a side distal from the first side and
proximate to the end face 152) with reflective dielectric thin film
110. It is contemplated that absorptive coating 112 can include
multiple metal coatings with the same or different materials. It is
also contemplated that the reflective dielectric thin film 110 can
include multiple layers of the same or different materials. The FPI
can be connected to the end face 152 on the reflective dielectric
thin film 110 side. The reflective dielectric thin film 110 serves
as a mirror for the FPI. The absorptive coating 112 functions both
as a second mirror for the FPI and as an absorptive element that
absorbs an incident signal 116 and conducts heat to the silicon
pillar. The choice of absorptive coating 112 material and thickness
may be optimized to adjust the thermal properties of the combined
pillar and its wavelength dependent absorptivity. Signals that
penetrate through the absorptive coating 112 can also contribute to
the heating of the silicon pillar but do not perturb its operation
as an FPI. The heating of the silicon pillar increases the optical
length if the FPI via increasing both the refractive index and the
physical length of the silicon pillar through, respectively, the
thermal optic effect and the thermal expansion of silicon material.
The change in the optical length of FPI shifts the interferometric
fringes in the reflection spectrum of the FPI, which can be
measured by the light guided to the FPI through the optical fiber
102. The two mirrors (e.g., reflective dielectric thin film 110,
absorptive coating 112) with high reflectivity can yield a large
Finesse and narrow spectral features of the FPI, leading to high
resolution in determining the wavelength shift of the fringes. The
transmitted light 140 (e.g., light from a light source 126) should
have wavelengths transparent to the silicon pillar. In one
implementation, the transmitted light 140 wavelength is the 1550 nm
window where a large of low-cost fiber-optic components and devices
are available.
The fiber optic sensor 100 parameters, including the type and
thickness of the absorptive coating(s) 112, the thickness of the
silicon pillar, the reflectivity of the reflective dielectric thin
film 110, can be optimized for different applications. For example,
the absorptive coating 112 may include a gold metal coating, which
serves as an excellent mirror with a reflectivity >95% over a
broad wavelength range. A few micrometer gold coating can serve as
good absorptive element from x-ray to the short end of the visible.
Another choice of the metal may include platinum, which is commonly
used in electronic resistive bolometers as the absorptive element.
The thickness of the silicon pillar and the metal film can be
designed to maximize the sensitivity in terms of minimally
resolvable power of the incident signal.
Another function of the optical fiber 102 is to provide a thermal
reservoir to the FPI. The optical fiber 102 can include specialty
fibers to tune the performance of the bolometer. For example, the
optical fiber 102 may include microstructured fibers, photonic
crystal fibers, or side-hole fibers to tune the speed and
sensitivity of the bolometer. For example, FIGS. 1K and 1L
illustrates a specific optical fiber 102 that includes a side hole
fiber, with cladding 106 that has at least one cavity 114 (e.g., an
empty hole or space) along at least a portion of the length of the
optical fiber 102. The at least one cavity 114 can be disposed
proximate to an end face in the optical fiber 102. The cavities 114
effectively reduces the thermal conductivity of the fiber optic
sensor 100. Because of the cavities 114, the heat transfer from the
FPI to the optical fiber 102 becomes less efficient; therefore,
increasing the sensitivity of the bolometer but reducing its speed.
In some implementations, as shown in FIG. 1M, the cavity 114 can be
filled with a thermal conductor 118 (e.g., thermal conductive
medium), such as metal, that can help to dissipate the heat from
the FPI to the optical fiber 102 to reduce the sensitivity and
increase the speed of the fiber optic sensor 100.
FIG. 1M illustrates a specific embodiment where the fiber optic
sensor 100 includes a mirror coating 122 dedicated as a highly
reflective mirror for the FPI. The mirror coating 122 can be
disposed between the absorptive coating 112 for signal absorption
and the silicon pillar (e.g., silicon layer 108). In
implementations, the mirror coating 122 may include a metal (e.g.,
gold) or a multilayer dielectric coating, similar to the reflective
dielectric thin film 110 on the other side of the FPI. In instances
where a mirror coating 122 includes a metal, a small amount of the
light power can be absorbed to heat up the FPI (e.g., .about.2% of
light power can be absorbed by a mirror coating 122 including gold
at about 1550 nm). As a result, the light power is maintained at a
constant level. Otherwise, light power fluctuations may change the
FPI temperature and can be mistakenly explained as the changes of
the radiation signal. When a dielectric mirror coating 122 is used,
the absorption of the light power may be smaller than a metal
mirror coating 122. In the specific embodiment illustrated in FIG.
1M, the absorptive coating 112 can function mainly for the
absorption of the incident signal 116. In addition, materials other
than metal can also be used for the absorptive coating 112. With
the absorptive coating 112, in general, any material that can
convert the power flux into heat may be used. For example, the
absorptive coating 112 may include carbon, which can be used as an
absorber for radio frequency waves.
FIG. 1N illustrates an embodiment where the fiber optic sensor 100
includes a graded-index (GRIN) lens 176 disposed between the
optical fiber 102 and the silicon layer 108. The profile of the
light beam in a regular single mode fiber is close to that of a
Gaussian beam and the mode field diameter is usually small (e.g.,
less than 15 .mu.m). As the light beam exits the optical fiber 102
and travels into the FPI, the diffraction, which can be more
prominent for smaller light beam, may limit the finesse of the FPI.
The fiber optic sensor 100 illustrated in FIG. 1N can increase the
size of the light beam in the FPI and increase the finesse of the
FPI using a graded-index (GRIN) lens 176, which may also include a
short-section of GRIN multimode fiber (MMF), that is inserted
between the silicon layer 108 and the optical fiber 102. The GRIN
lens 176 (and/or the MMF) functions as a light collimator, which
can expand the light beam size to reduce light diffraction and
increase finesse.
FIG. 1O illustrates an embodiment where the fiber optic sensor 100
includes an optical fiber 102 with an expanded core 178 disposed
proximate to and/or abutted against the silicon layer 108 and/or
reflective dielectric thin film 110. In this embodiment, the core
104 of the optical fiber 102 can be gradually expanded resulting in
an expanded core 178. As the core 104 is gradually expanded toward
the expanded core 178, the light beam profile can be gradually
enlarged. In implementations, the expanded core 178 can be obtained
by locally heating the fiber to a high temperature.
Example Processes
The following discussion describes example techniques for utilizing
a fiber optic sensor and fiber optic sensing system, such as the
fiber optic sensor 100 and fiber optic sensing system 130 described
in FIGS. 1A through 1M. FIG. 2A depicts an example process 200 for
using a bolometer device 100.
As shown in FIG. 2A, a light source is caused to transmit light
through a fiber optic to a fiber optic sensor (Block 202). In this
implementation, controller 132 can cause light source 126 to
transmit light (e.g., transmitted light 140) through optical fiber
102 and circulator 124 to fiber optic sensor 100. Controller 132
can control the duration and intensity that the light source 126
transmits the transmitted light 140. In some specific
implementations, controller 132 can cause heating light source 146
to transmit heating light 142 through the optical fiber 102 to the
fiber optic sensor 100 and the silicon layer 108 for providing
heat.
Reflected light from the fiber optic sensor is received using a
spectrometer (Block 204). The spectrometer 128 can receive the
reflected light 162 and associated spectra, which can be recorded
and/or analyzed by spectrometer 128 and/or controller 132.
An output from the spectrometer is analyzed based on the received
reflected light (Block 206). In implementations, the controller 132
and/or the spectrometer 128 can analyze the reflected light 162 and
the spectra to determine a wavelength shift in the spectra, which
indicates a change in temperature. A variety of methods may be
utilized to analyze and/or determine the wavelength shift in the
spectra and for tracking the average wavelength. In a specific
embodiment, analyzing an output from the spectrometer based on
received reflected light can include using an average wavelength
tracking method to further increase the resolution of wavelength
and/or measurand.
The following discussion describes example techniques for
fabricating a fiber optic sensor, such as the fiber optic sensor
100 described in FIGS. 1A through 1M. FIG. 2B depicts an example
process 300 for fabricating the fiber optic sensor 100. FIGS. 3A
through 3E illustrate a section an exemplary fiber optic sensor 100
during fabrication (such as the fiber optic sensors 100 described
in FIGS. 1A through 1M).
FIG. 2B depicts an example process 300 for fabricating a fiber
optic sensor 100. As shown in FIG. 2B, a silicon pillar is formed
on a silicon substrate (Block 302). FIG. 3A illustrates forming at
least one silicon pillar 150 that will function as a sensor head
for the fiber optic sensor 100. In one specific implementation, a
double-side-polished silicon wafer (e.g., 200 .mu.m thick) can be
bonded on top of another larger silicon wafer using a layer of
photoresist 154. The larger silicon wafer can function as a silicon
substrate 148 to facilitate the fabrication and later as a support
for the fabricated silicon pillar 150. Then another layer of
photoresist 154 can be coated on the top of the
double-side-polished silicon wafer and patterned accordingly using
photolithography techniques. The patterned top silicon layer can be
etched all the way to the silicon substrate 148 and the second
layer of photoresist 154 using, for example,
deep-reactive-ion-etching, leaving the upstanding silicon pillar(s)
150 attached to the silicon substrate 148.
Then, a thin film adhesive is formed on a glass substrate (Block
304). In some specific embodiments, such as the one illustrated in
FIG. 3B, an adhesive 156 including a thin film of UV-curable glue
can be spin-coated on a piece of glass substrate 158. It is
contemplated that forming a thin film adhesive 156 may include
using other adhesives and/or other methods for depositing and/or
forming the adhesive 156. In embodiments, the adhesive 156 may
include a thin film adhesive (e.g., UV-curable glue, an epoxy-based
adhesive, and/or a gel-based adhesive).
Shown in FIG. 3C, an end face of an optical fiber is pressed onto
the thin film adhesive (Block 306). In implementations, the
adhesive 156 can be transferred to a cleaved and cleaned end face
152 of an optical fiber 102 by pressing the end face 152 of the
optical fiber 102 to the adhesive 156 on the glass substrate 158.
Subsequently, the end face 152 and the adhesive 156 can be released
from the glass substrate 158.
As illustrated in FIGS. 3D and 3E, the optical thin film adhesive
on the end face is placed onto the silicon pillar to provide the
fiber optic sensor (Block 308). The optical fiber 102 with the
silicon pillar 150 (silicon layer 108) attached can be lifted from
the silicon substrate 148 and the second layer of photoresist 154.
Further, fabrication of the fiber optic sensor 100 may include
cleaning residual photoresist 154 from the end of the silicon
pillar 150 (e.g., with alcohol). Due to the ultra-thin thickness of
the residual photoresist 154 between the optical fiber 102 and the
silicon pillar 150/silicon layer 108, the reflection spectrum of
the FP cavity within the fiber optic sensor 100 is not
affected.
Then, the thin film adhesive is cured (Block 310). In
implementations, the adhesive 156 can be cured, for example, by UV
irradiation (e.g., UV light 160). It is contemplated that other
bonding technology may be implemented to mount the silicon layer
108 (or other material) to the end face 152 of the optical fiber
102, such as physical contact bonding.
It is contemplated that the above steps may be repeated to form a
fiber optic sensor 100 with cascaded Fabry-Perot cavities. For
example, the optical fiber 102 with the silicon pillar 150 may be
further pressed onto a second adhesive 166 on a glass substrate,
placed on a second silicon pillar (e.g., second silicon layer 168),
and cured using UV light, to form a fiber optic sensor 100 with two
Fabry-Perot interferometers. Further Fabry-Perot interferometers
may be fabricated by repeating the above steps.
As described above, a first side (e.g., side or surface distal to
the end face 152) of the silicon pillar 150/silicon layer 108 can
be coated with an absorptive layer (e.g., coating) 112 and a second
side (e.g., side or surface proximal or adjacent to the end face
152) of the silicon pillar 150/silicon layer 108 can be coated with
a reflective dielectric thin film 110. Thus, as described above,
suitable techniques can be employed to deposit (e.g., coat) the
respective surfaces of the silicon pillar 150/silicon layer 108
with the respective absorptive layer (e.g., coating) 112 and/or the
reflective dielectric thin film 110.
FIG. 2C illustrates an exemplary process 400 for fabricating a
bolometer device 100. As shown in FIG. 2C, an optical fiber
including a reflective dielectric film on a cleaved endface of the
optical fiber is received (Block 402). FIG. 4A illustrates
receiving at least one optical fiber 102 having a reflective
dielectric thin film 110 disposed on a cleaved end of the optical
fiber 102. In some instances, multiple optical fibers 102 can
include the reflective dielectric thin film 110 for batch
processes.
FIG. 2C illustrates forming at least one silicon pillar (Block
404). In implementations, forming at least one silicon pillar 150
(e.g., silicon layer 108) can include forming a silicon pillar
150/silicon layer 108 that will function as a sensor head for the
fiber optic bolometer device 100. In one specific implementation,
the at least one silicon pillar 150 can be prepared by patterning
and DRIE etching a silicon wafer 148 with a selected thickness
after which each silicon pillar 150 can be attached to the silicon
wafer 148 main frame through very thin arms (see FIG. 4B). The
patterned top layer of the silicon wafer 148 can be etched all the
way through the silicon substrate 148 using, for example,
deep-reactive-ion-etching (DRIE), leaving the upstanding silicon
pillar(s) 150 attached to the silicon substrate 148. In some
specific embodiments, the reflective dielectric thin film 110 can
be deposited on the surface of the silicon wafer 148 instead of the
optical fiber endface 152 before patterning and dry etching the
silicon wafer 148.
Then, a thin film adhesive is placed on the reflective dielectric
thin film (Block 406). In some specific embodiments, such as the
one illustrated in FIG. 4C, an adhesive 156 including a thin film
of UV-curable glue can be spin-coated on a piece of glass substrate
158. It is contemplated that forming a thin film adhesive 156 may
include using other adhesives and/or other methods for depositing
and/or forming the adhesive 156. In embodiments, the adhesive 156
may include a thin film adhesive (e.g., UV-curable glue, an
epoxy-based adhesive, and/or a gel-based adhesive). The optical
fiber 102 can then be dipped onto the adhesive 156 and the
substrate 158 and removed.
As illustrated in FIG. 2C, the adhesive on the end face of the
optical fiber is placed onto the silicon pillar (Block 408). As
shown in FIG. 4D, the optical fiber 102 with the silicon pillar 150
(e.g., silicon layer 108) attached can be lifted from the silicon
substrate 148. In a specific implementation, the optical fiber 102
with the adhesive 156 (e.g., UV glue) can be pushed and/or placed
onto the silicon pillar 150 and exposed to UV light to cure the
adhesive 156 so that the silicon pillar 150 is firmly attached to
the optical fiber endface 152, after which the optical fiber 102
can be lifted up with the silicon pillar 150, where the silicon
pillar 150 breaks/detaches from the silicon wafer 150 at the thin
arm formed during initial etching.
Next, the silicon pillar is etched (Block 410). In this step,
etching the silicon pillar 150 can include can include fine-tuning
the thickness of the silicon pillar 150 using, for example, wet
etching (e.g., wet enchant 172 as shown in FIG. 4E), to control the
wavelength positions of the interferometeric fringes of the
Fabry-Perot interferometer within the bolometer device 100.
As illustrated in FIG. 2C, an absorptive coating is then placed on
the silicon pillar (Block 412). In one specific embodiment, as
shown in FIG. 4F, placing an absorptive coating 112 on the silicon
pillar 150 can include applying a gold coating on the silicon
pillar 150 on the optical fiber endface 152 using, for example,
plating, sputtering, and/or other deposition processes.
It is contemplated that the above steps may be repeated to form a
fiber optic sensor 100 with cascaded Fabry-Perot cavities. For
example, the optical fiber 102 with the silicon pillar 150 may be
further pressed onto a second adhesive 166 on a glass substrate,
placed on a second silicon pillar (e.g., second silicon layer 168),
and cured using UV light, to form a fiber optic sensor 100 with two
Fabry-Perot interferometers. Further Fabry-Perot interferometers
may be fabricated by repeating the above steps.
Multiple bolometers can be assembled to form a 2-D array and
several bolometers can share a single tunable laser for signal
demodulation, as illustrated in FIG. 3. In this example, a tunable
laser, such as a distributed feedback (DFB) laser diode or an
external cavity semiconductor laser, is used and the bolometers
that share this laser are made to have their spectral notches
within the wavelength tunable range of the laser. The light from
the laser is split into each of the bolometers through a
fiber-optic coupler and the reflected light from each bolometer is
guided to a photodetector through a fiber-optic coupler or
fiber-optic circulator. A separate photodetector is needed for each
bolometer. The wavelength of the tunable laser is tuned and the
waveform in the time domain from the photodetector is converted to
the wavelength-domain to obtain the reflection spectrum of the
bolometer.
FIG. 2D depicts an example process 500 for demodulating the fiber
optic bolometer device 100. Multiple fiber optic bolometer devices
100 can be assembled to form a 2-D bolometer array 170, and several
fiber optic bolometer devices 100 can share a single tunable laser
(e.g., light source 126) for signal demodulation, as illustrated in
FIG. 5. As shown in FIG. 2D, a light source is caused to transmit
light through a fiber optic to a fiber optic bolometer device array
(Block 502). In this implementation, controller 132 can cause light
source 126 to transmit light (e.g., transmitted light 140) through
optical fiber 102 and circulator 124 to fiber optic bolometer
device array 170. Controller 132 can control the duration and
intensity that the light source 126 transmits the transmitted light
140. In a specific implementation, a light source 126 can include a
tunable laser, such as a distributed feedback (DFB) laser diode or
an external cavity semiconductor laser, and the fiber optic
bolometer devices 100 in the fiber optic bolometer device array 170
that share the laser diode are configured to have their spectral
notches within the wavelength tunable range of the laser diode. The
transmitted light 140 from the light source 126 is split into each
of the bolometers (four are illustrated in FIG. 5) through a
fiber-optic coupler 124.
Reflected light from the fiber optic sensor is received using at
least one photodiode (Block 504). In embodiments, the at least one
photodiode 174 can be included in a spectrometer 128. The
spectrometer 128 can receive reflected light 162 and associated
spectra from each of the fiber optic bolometer devices 100, which
can be recorded and/or analyzed by spectrometer 128 and/or
controller 132. The reflected light 162 from each fiber optic
bolometer device 100 can be guided to a photodetector 174 through a
fiber-optic coupler or fiber-optic circulator 124. In general, a
separate photodetector 174 is needed for each fiber optic bolometer
device 100.
An output from the at least one photodiode is analyzed based on the
received reflected light (Block 506). In implementations, the
controller 132, the at least one photodiode 174, and/or the
spectrometer 128 can analyze the reflected light 162 and the
spectra to determine a wavelength shift in the spectra, which
indicates a change in temperature. A variety of methods may be
utilized to analyze and/or determine the wavelength shift in the
spectra and for tracking the average wavelength. In a specific
embodiment, analyzing an output from the spectrometer based on
received reflected light can include using an average wavelength
tracking method to further increase the resolution of wavelength
and/or measurand. The wavelength of the light source 126 (e.g.,
tunable laser) can be tuned and the waveform in the time domain
from each photodetector 174 can be converted to the
wavelength-domain to obtain the reflection spectrum of each fiber
optic bolometer device 100.
CONCLUSION
Although the subject matter has been described in language specific
to structural features and/or process operations, it is to be
understood that the subject matter defined in the appended claims
is not necessarily limited to the specific features or acts
described above. Rather, the specific features and acts described
above are disclosed as example forms of implementing the
claims.
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